1. Field of the Invention
This invention relates to a system for sensing the presence or absence of a liquid and more particularly to the adaptation of a system which is used to monitor the effectiveness of the corrosion inhibition character of a vehicular coolant liquid to additionally sense the presence or absence of such liquid without deleteriously effecting its monitoring performance.
2. Description of the Prior Art
A great deal of effort is expended throughout industry in an attempt to prevent, or at least mitigate, corrosion of metal components. In the automotive industry, for instance, among the various preventive measures that are commonly taken is the use of additives in vehicular coolant systems to inhibit corrosion. Commerically available permanent antifreeze used in automotive cooling systems inhibits corrosion of the radiator and allied metallic components until it chemically breaks down with age, is diluted with water or is contaminated in some way. Recently systems have been developed to monitor the effectiveness of the corrosion inhibition characteristic of such coolant material whereby visual indication is provided upon the loss of this characteristic so that the coolant liquid can be replaced before any corrosion occurs but not before replacement is required. Examples of such systems are found in copending applications Ser. No. 879,188 filed Feb. 21, 1978 and Ser. No. 866,074 filed Dec. 30, 1977, both assigned to the assignee of the present invention. In these systems a potential measuring circuit employing at least two electrochemical electrodes composed of dissimilar metals are located so as to be immersed and in contact with coolant liquid. When the inhibiting characteristic is effective a first range of electrical potential exists between the electrodes; however, when the inhibiting characteristic becomes ineffective for any reason, a second range of electrical potential exists therebetween. Upon reaching a threshold level, as the potential moves into the second range, indicating means are activated to provide suitable indication of the condition. For instance a light mounted in the dashboard can be actuated when the coolant liquid becomes corrosive.
In addition to systems for monitoring the effectiveness of the inhibition characteristics of coolant liquid it would also be advantageous to provide means for indicating whether an adequate supply of coolant liquid exists in the cooling system to perform its intended cooling function. A common way to provide this type of information is to mount a liquid level sensor at a selected position in the cooling system so that when the sensor is inundated it is in a first operating mode and when it is not so inundated it is in a second operating mode. Thus when the liquid level drops below the sensor and the sensor goes into its second operating mode, an alarm is given indicating the existance of an inadequate supply of coolant liquid.
However, if the monitoring systems of the above mentioned applications are employed, then a sensor is already disposed in the cooling system so it would be very desirable to use this same sensor to provide the level sensing function as well as the function of monitoring the effectiveness of the corrosion inhibition characteristic of the fluid. Given the existence of a sensor comprising one or more electrodes a conventional approach for sensing the presence or absence of an electrically conductive liquid would entail passing current between an electrode and ground so that the potential measured between the electrode and ground would serve to indicate the presence or absence of liquid. In such systems, in order to avoid false tripping due to noise a reasonably large trip potential is desirable, for example, in the order of one volt. However, to avoid false high level indication due to leakage through liquid clinging to the insulator header mounting the electrode, a reasonably low trip resistance is necessary, for example, in the order of 5000-10,000 ohms. One volt at 10,000 ohms results in 100 microamperes of current which, if used with the monitoring system set forth in the aforementioned applications, would not be acceptable due to polarization it would cause. Even where a separate level sensor is employed, that is independently of the corrosion sensing function, such current levels can cause unacceptable electrolytic reactions. It has been found that for use with the above referenced corrosion sensor systems, as little as one microampere produces unacceptable polarization.